Claudin 5 (CLDN5) a member of the claudin family. Claudins are integral membrane proteins and components of tight junction strands. Homozygous mutant neonates gradually cease movement and die within 10 hours after birth. (Nitta et al. 2003). In the adult mouse brain, CLDN5 is expressed in all major forebrain subdivisions: the neocortex, hippocampus, basal ganglia, amygdala/basal forebrain, and olfactory bulb, as well as all other CNS regions (Maynard et al. 2003). CLDN5 is expressed in cultured mouse brain embryonic cells and in freshly isolated MBEC as early as embryonic Day 7. In situ hybridization and immunocytochemical analyses revealed the presence of the CLDN5 mRNA and its protein product in brain capillary endothelial cells, as well as in a subset of other endothelial and epithelial cells (Chen et al. 1998). In the brain and lung, immunofluorescence microscopy have shown that CLDN5 is exclusively concentrated at cell-cell borders of endothelial cells of all segments of blood vessels, but not at those of epithelial cells (Morita et al. 1999). Endothelial tight junctions are an important functional part of the formation of the blood brain barrier, and CLDN5 has been shown to be a critical determinant of blood brain barrier permeability (Matter and Balda 2003; Nitta et al. 2003; Stamatovic et al. 2008).
Functional mouse CLDN5 promoter sequences have been identified and analyzed (Burek and Forster 2009). In this paper, the authors perform luciferase reporter assays in a brain microvascular endothelial cell line, but do not report any in vivo expression data. A 1.5 kb huma CLDN5 promoter sequence was tested for expression in bovine retinal endothelial cells (Felinski et al. 2008).
There exists a significant need for minimal human promoter elements which are capable of driving expression in specific cell types and/or in specific regions of the brain. Identification of minimal elements required for adequate expression and specificity will allow ease of use in expression constructs.